Step-index-type plastic-clad optical fibers (hereinafter referred to as PCF) comprising a quartz core and a plastic clad are relatively inexpensive, and, when compared with plastic optical fibers of which the core and the clad are both made of plastics, have higher light transmittance and better bondability to light-emitting and light-receiving devices and are easier to handle. For their such characteristics, PCF are much used as optical fibers for middle-range and short-range light information transmission and those in light guides.
For example, PCF described in Japanese Patent Application Laid-Open (JP-A) Hei-5-271350, of which the core is made of pure quartz and the clad is of UV-cured fluorine resin, is usable as a light information-transmitting material for intra-system communication, inter-system communication, mobile communication, intra-building communication and the like. As its clad is made of fluorine resin having high hardness, the PCF is compatible with crimp-style connectors, and field working of the PCF including connector application thereto is easy.
However, though having such excellent handlability, workability, connector applicability and bondability and being inexpensive, those conventional PCF have a large numerical aperture (NA) of from 0.37 to 0.40 and therefore their transmission band is narrow. Accordingly, for light information-transmitting materials for wide-band information transmission systems of ATM-LAN (Asynchronous Transfer Mode-LAN), high-speed Ethernet and the like, the conventional PCF are unsatisfactory as their transmission band is too narrow.
On the other hand, graded-index-type and single-mode-type optical fibers of quartz, which are known as materials for high-speed information transmission, have a wide transmission band, but require troublesome working and connector application and even complicated material designing and clad constitution. Therefore, their production costs are difficult to reduce.
Given that situation, the object of the present invention is to improve PCF which are easy to work, to which connector application is easy and for which material designing and clad constitution is easy, to thereby broaden their band with no disadvantages including contact loss for simple crimp-style connectors, bending loss and even light transmittance loss in temperature change.